1. Field of the Invention
The present invention relates to a process for the preparation of ferromagnetic metallic particles for magnetic recording. Particularly, it relates to a process for the preparation of ferromagnetic metallic particles for magnetic recording which are improved in the dispersibility, particularly the easiness of dispersion, the magnetic stability, and the magnetic properties, particularly the coercivity, and which can satisfy the demands for finer ferromagnetic metallic particles. The process is shortened and simplified in the processing steps and is economically advantageous.
2. Prior Art
With the purpose of improving the magnetic recording density and the playback output, recently, coated type magnetic recording media using ferromagnetic metallic particles comprising pure iron or containing iron as a main component have been put to practical use.
However, such ferromagnetic metallic particles, having poorer magnetic stability, deteriorate more easily with the lapse of time than magnetic iron oxide particles and they are liable to agglomerate to result in poor dispersion, though they have an advantage of higher magnetic recording density.
Further, in a recent tendency toward the higher magnetic recording density, finer ferromagnetic metallic particles are desired. But the poorer magnetic stability and dispersibility of finer ferromagnetic metallic particles have become more and more serious problems.
Although various proposals including the incorporation of aluminum or other elements into ferromagnetic metallic particles have been made in order to solve these problems, the problem of poor dispersion has not been solved as yet. Further, from the standpoint of both cost and productivity, it is an important factor to shorten the dispersion time in the preparation of coated type magnetic recording media. That is, ferromagnetic metallic particles which have the easiness of dispersion are more desirable. The term "the easiness of dispersion" used in this specification refers to the time required for gloss and squareness of the coated sheet to reach respective equilibria when the particles are dispersed by a sand mill into a magnetic paint. The longer this time, the poorer the easiness of dispersion. Recently, it is particularly strongly required to improve this easiness of dispersion.
For example, Japanese Patent Laid-Open Gazette No. Sho. 57-73105 discloses a process for the preparation of ferromagnetic metallic particles by using iron oxide hydrate containing aluminum in a solid solution state. However, this process necessitates very restricted conditions including an excess alkali concentration of 0.4N or below and a liquid temperature of 40.degree. to 50.degree. C., so that the process is unsuitable for the preparation of fine ferromagnetic metallic particles having high coercivity, and the obtained particles are not improved in the dispersibility.
Further, Japanese Patent Laid-Open Gazette No. Sho. 64-33019 discloses a process for the preparation of acicular iron oxide hydrate which comprises reacting an aqueous solution of a ferrous salt with an aqueous solution of an alkali hydrate to give an aqueous suspension of ferrous hydrate and introducing an oxygen-containing gas into the suspension, while adding an aluminum compound thereinto. However, the ferromagnetic metallic particles prepared by this process are not improved in the dispersibility. Further, the yield of aluminum is low resulting in low productivity, and the control of powder properties is difficult. Furthermore, the particles are apt to become dendritic to thereby worsen the switching field distribution (SFD).
Furthermore, Japanese Patent Laid-Open Gazette No. Sho. 64-140005 discloses a process which comprises adding an aluminum compound and a ferrous salt to an aqueous suspension of iron oxide hydrate, oxidizing the obtained mixture, and coating the surface of the obtained particles by Ni, a water-soluble carboxylic acid, and a silicon compound. However, the ferromagnetic metallic particles prepared by this process have a problem of hindering the effect of aluminum which contributes to the dispersibility of the particles. Further, this process must be conducted in a pH range of 8 to 12, so that a part or most of aluminum precipitates as hydroxide not only on the surface of particles but also at other places, so that the obtained particles are not improved in the dispersibility. Further, the process has another problem of necessitating very complicated stages, thus lowering the productivity.